Some of it is on my local patch and there are some nice off road bits now. If you were to do it from west to east you would have a nice descent from Ben Bowie down the recently completed Killoeter Escarpment path and on down the Stoney Mollan into Balloch. This section is shared with the Three Lochs Way and it’s described on their website. I don’t know much about the rest of the JMW.
I can’t imagine the JMW being anything like as technical as the WHW. Good call on the Drunkie route for RRW. The bit past the Menteiths Hills is great in the dry but it’s been wet and there’s been forestry work churning up bits too in recent years.